This invention relates generally to apparatus and methods for removing solder ball connections from a backside of a circuit, such as a flip-chip package having elements attached by solder ball connections, and more specifically, to methods and apparatus for reworking such circuits having one or more defects related to the solder ball connections.
Circuit elements are often attached to a support board or circuit by solder ball connections. As will be appreciated by those skilled in the art, if one or more of the solder ball connections receives too much solder or somehow the solder is smeared or otherwise disturbed before the solder hardens, a solder connection may be created between two or more adjacent solder ball connections thereby causing a short and subsequent rejection of the circuit for being defective. Such defects historically have resulted in a yield ratio of about 99.2%, which although a seemingly high yield rate, still results in thousands of dollars of rejects every day. Increasing the yield rate by as little as a half of a point would typically cut the losses due to such defective solder ball connection to less than half of that presently experienced.
In the preferred embodiment, the present invention provides methods and apparatus for removing solder ball connections from electrical circuits. The methods and apparatus comprise a solder ball-removing tool that has a multiplicity of raised ribs that define a contact surface. The raised ribs may take substantially any shape, but one effective embodiment is simply paralleled raised ribs that are spaced from each other at a distance somewhat greater than the average diameter of a solder ball connection. The raised ribs define a contact surface and are made of a heat conductive material, such as for example, aluminum, copper or iron or alloys of such metals. Thus, any suitable heat conductive material is satisfactory so long as the melting point of the material is well above that of the solder that is to be removed.
Also included is a heat source for heating at least a portion, if not all, of the multiplicity of raised ribs to a temperature that is sufficiently great to melt the solder connections, yet less than the melting point of the heat conductive ribs. Also included is a support structure for supporting the solder ball-removing tool at an angle tilted to the horizontal. Alternately, the support structure may support the solder ball-removing tool horizontally, while the raised ribs will be higher at one end and shorter at the other end to create the tilt effect.
Also as will be appreciated by those skilled in the art, a suitable tool for picking up and holding the individual chips to be reworked on the soldering tool may comprise a vacuum holding tool such that the circuit can be placed against the heated contact surface of the raised ribs. The circuit is then held against the tilted surface of the contact surface until it is heated to a level that the solder ball connections begin to melt. At that point, the circuit is moved back and forth across the edges of the raised ribs to knock off the solder, which then flows down the ribs to a collecting pan or container at the bottom of the lower end of the raised ribs.
According to a preferred embodiment, a flux material such as WS-600, commercially available from the Alphametals Corporation in Singapore, is used. To assure a full removal of the solder ball connections, it is advisable to repeat the applying of the flux and heating and rubbing steps at least one more time.